Closed-cell polyurethane foams are widely used for insulation purposes in building construction and in the manufacture of energy efficient electrical appliances. In the construction industry, polyurethane (poly-isocyanurate) board stock is used in roofing and siding for its insulation and load-carrying capabilities. Poured and sprayed polyurethane foams are also used in construction. Sprayed polyurethane foams are widely used for insulating large structures such as storage tanks, etc. Pour-in-place polyurethane foams are used, for example, in appliances such as refrigerators and freezers plus they are used in making refrigerated trucks and railcars.
All of these various types of polyurethane foams require expansion agents (blowing agents) for their manufacture. Insulating foams depend on the use of halocarbon blowing agents, not only to foam the polymer, but primarily for their low vapor thermal conductivity, a very important characteristic for insulation value. Historically, polyurethane foams are made with CFC-11 as the primary blowing agent.
Another important type of insulating foam is phenolic foam. These foams, which have very attractive flammability characteristics, are generally made with CFC-11 and CFC-113 blowing agents.
As modern electronic circuit boards evolve toward increased circuit and component densities, thorough board cleaning after soldering becomes a more important criterion. Current industrial processes for soldering electronic components to circuit boards involve coating the entire circuit side of the board with flux and thereafter passing the flux-coated board over preheaters and through molten solder. The flux cleans the conductive metal parts and promotes solder fusion. Commonly used solder fluxes generally consist of rosin, either used alone or with activating additives, such as amine hydrochlorides and oxalic acid derivatives.
After soldering, which thermally degrades part of the rosin, the flux-residues are often removed from the circuit boards with an organic solvent. The requirements for such solvents are very stringent. Defluxing solvents should have the following characteristics: Have a low boiling point, be nonflammable, have low toxicity and have high solvency power, so that flux and flux-residues can be removed without damaging the substrate being cleaned.
While boiling point, flammability and solvent power characteristics can be adjusted by preparing solvent mixtures, these mixtures are often unsatisfactory because they fractionate to an undesirable degree during use. Such solvent mixtures also fractionate during solvent distillation, which makes it virtually impossible to recover a solvent mixture with the original composition.
On the other hand, azeotrope-like mixtures, with their essentially constant compositions, have been found to be very useful for these applications. Azeotrope-like mixtures, for all practical purposes, do not fractionate on evaporation or boiling. These characteristics are also important when using solvent compositions to remove solder fluxes and flux-residues from printed circuit boards. Preferential evaporation of the more volatile solvent mixture components would occur if the mixtures were not azeotrope-like. This could result in mixtures with changed compositions and less-desirable solvency properties, such as lower rosin flux solvency and lower inertness toward the electrical components being cleaned. This azeotrope-like character is also desirable in vapor degreasing operations, where redistilled solvent is generally employed for final rinse cleaning.
Many solvent compositions used industrially for cleaning electronic circuit boards and for general metal, plastic and glass cleaning are based upon CFC-113.
Refrigeration systems such as centrifugal chillers are used to cool water, food, etc. These systems presently may use CFC-11 as the refrigerant.
In the early 1970s, concern began to be expressed that the stratospheric ozone layer (which provides protection against penetration of the earth's atmosphere by ultraviolet radiation) was being depleted chlorine atoms introduced to the atmosphere from the release of fully halogenated chlorofluorocarbons. These chlorofluorocarbons are used as propellants in aerosols, as blowing agents for foams, as refrigerants and as cleaning/drying solvent systems. Because of the great chemical stability of fully halogenated chlorofluorocarbons, according to the ozone depletion theory, these compounds do not decompose in the earth's atmosphere but reach the stratosphere where they slowly degrade liberating chlorine atoms which in turn react with the ozone.
Concern reached such a level that in 1978 the U.S. Environmental Protection Agency (EPA) placed a ban on nonessential uses of fully halogenated chlorofluorocarbons as aerosol propellants. This ban resulted in a dramatic shift in the U.S. away from chlorofluorocarbon propellants (except for exempted uses) to primarily hydrocarbon propellants. However, since the rest of the world did not join the U.S. in this aerosol ban, the net result has been to shift the uses of chlorofluorocarbons in aerosols out of the U.S., but not to permanently reduce the world-wide total chlorofluorocarbon production, as sought. In fact, in the last few years the total amount of chlorofluorocarbons manufactured worldwide has exceeded the level produced in 1978 (before the U.S. ban).
During the period of 1978-1987, much research was conducted to study the ozone depletion theory. Because of the complexity of atmospheric chemistry, many questions relating to this theory remained unanswered. However, assuming the theory to be valid, the health risks which would result from depletion of the ozone layer are significant. This, coupled with the fact that world-wide production of chlorofluorocarbons has increased, has resulted in international efforts to reduce chlorofluorocarbon use. Particularly, in September, 1987, the United Nations through its Environment Programme (UNEP) issued a tentative proposal calling for a 50 percent reduction in world-wide production of fully halogenated chlorofluorocarbons by the year 1998. This proposal was ratified on Jan. 1, 1989, and it is scheduled to become effective on Jul. 1, 1989.
Because of this proposed reduction in availability of fully halogenated chlorofluorocarbons such as CFC-11, CFC-12 and CFC-113, alternative, more environmentally acceptable, products are urgently needed.
As early as the 1970s with the initial emergence of the ozone depletion theory, it was known that the introduction of hydrogen into previously fully halogenated chlorofluorocarbons markedly reduced the chemical stability of these compounds. Hence, these now destabilized compounds would be expected to degrade in the atmosphere and not reach the stratosphere and the ozone layer. The accompanying Table lists the ozone depletion potential for a variety of fully and partially halogenated halocarbons. Greenhouse potential data (potential for reflecting infrared radiation (heat) back to earth and thereby raising the earth's surface temperature) are also shown.
______________________________________ OZONE DEPLETION AND GREENHOUSE POTENTIALS Ozone Depletion Greenhouse Blowing Agent Potential Potential ______________________________________ CFC-11 (CFCl.sub.3) 1.0 0.4 CFC-12 (CF.sub.2 Cl.sub.2) 1.0 1.0 HCFC-22 (CHF.sub.2 Cl) 0.05 0.07 HCFC-123 (CF.sub.3 CHCl.sub.2) 0.02 less than 0.1 HCFC-124 (CF.sub.3 CHFCl) 0.02 less than 0.1 HFC-134a (CF.sub.3 CH.sub.2 F) 0 less than 0.1 HCFC-141b (CFCl.sub.2 CH.sub.3) 0.1 less than 0.1 HCFC-142b (CF.sub.2 ClCH.sub.3) 0.06 less than 0.2 HFC-152a (CHF.sub.2 CH.sub.3) 0 less than 0.1 CFC-113 (CF.sub.2 Cl--CFCl.sub.2) 0.8 0.3-0.8 ______________________________________
Halocarbons such as HCFC-123, HCFC-123a and HCFC-141b are environmentally acceptable in that they theoretically have minimal effect on ozone depletion. (Although these values have not been calculated for HCFC-123a, it is estimated that they would be similar to those for HCFC-123.)
Unfortunately, as recognized in the art, it is not possible to predict the formation of azeotropes or azeotrope-like mixtures.
This fact obviously complicates the search for new azeotrope-like compositions, which have application in the field. Nevertheless, there is a constant effort in the art to discover new azeotrope-like compositions, which have desirable characteristics.
An objective of this invention is to provide ozone compatible novel azeotrope-like compositions based on 1,1-dichloro-2,2,2-trifluoroethane and 1,1-dichloro-1-fluoroethane which have good solvency power and other desirable properties for cleaning applications and are useful as foam blowing agents and as refrigerants.
Another object of the invention is to provide novel azeotrope-like solvents which are liquid at room temperature, will not fractionate under use-conditions and also have the foregoing advantages.
A further objective is to provide azeotrope-like compositions which are relatively nontoxic and nonflammable.